16 Elementary Biology. 



of the work done by its expenditure. The standard .of 

 measurement was first determined by Joule, by his discovery 

 of the mechanical equivalent of heat. That discovery may 

 be thus expressed : the amount of heat-energy necessary 

 to elevate the temperature of one pound of water through 

 i Fahr., if converted into ordinary motion, is capable of 

 lifting one pound of water to a height of 772 feet from the 

 earth's surface ; and, conversely, a pound of water falling 

 from a height of 772 feet has its temperature raised in 

 consequence i Fahr. If a foot-ponnd be the unit of 

 mechanical work, i.e. the amount of energy required to raise 

 one pound to the height of one foot from the earth's sur- 

 face, and a ponnd-degree be the unit of heat- work, i.e. the 

 amount of energy required to raise one pound of water i 

 Fahr., then a definite quantitative relationship is established 

 between mechanical energy, or energy of visible motion, 

 and heat-energy, viz. 772 foot-pounds are equivalent to i 

 pound-degree. 



All the available terrestrial energy is derived from certain 

 stores of potential energy, and also from various forms of 

 kinetic energy, which are constantly being manifested on 

 the earth's surface. These different sources may be classi- 

 fied as being either directly or indirectly available, the latter 

 class being that from which the direct sources are them- 

 selves derived. 



A. DIRECTLY AVAILABLE SOURCE OF ENERGY 

 (POTENTIAL). 



Fuel Under this term are embraced all substances 

 used in the working of machines, whether these machines 

 be animal, vegetal, physical, or chemical. One instance 

 may suffice by way of illustration. 



Given the elementary substances, carbon (C), hydrogen 

 (H), and oxygen (O), it is possible, by the expenditure of a 

 certain amount of kinetic energy in a manner which we 



